This invention relates generally to the processing of a substrate utilizing a plasma in the production of integrated circuits, and specifically relates to the determination of substrate RF bias parameters in a plasma processing system, such as one utilizing an electrostatic chuck to secure a substrate to a susceptor during processing.
Gas plasmas are widely used in a variety of integrated circuit fabrication processes, including plasma etching and plasma deposition applications, such as PECVD. Generally, plasmas are produced within a processing chamber by introducing a low-pressure process gas into the chamber and then directing electrical energy into the chamber for creating an electric field therein. The electric field creates an electron flow within the chamber which ionizes individual gas molecules by transferring kinetic energy to the molecules through individual electron-gas molecule collisions. The electrons are accelerated within the electric field, producing efficient ionization of the gas molecules. The ionized particles of the gas and the free electrons collectively form what is referred to as a gas plasma or discharge.
Gas plasmas are useful in a variety of different processes for forming integrated circuits. One commonly used plasma process is a plasma etch process wherein a layer of material is removed or xe2x80x9cetchedxe2x80x9d from a surface of a substrate. In an etch process, the ionized gas particles of the plasma are generally positively charged, and the substrate is negatively biased such that the positively ionized plasma particles are attracted to the substrate surface to bombard the surface and thereby etch the substrate surface. For example, a substrate might be etched to remove an undesirable material layer or coating on the substrate before another layer is deposited. Such a pre-deposition etch process is often referred to as etch cleaning of the substrate.
Other common plasma processes involve deposition, wherein a material layer is deposited upon the substrate. Chemical vapor deposition, or CVD, for example, generally involves the introduction of material gases into a processing chamber wherein the gases chemically interact and form a material layer or coating on the exposed substrate surface. A gas plasma can be utilized to enhance the chemical interaction and the process. Consequently, such a CVD deposition process utilizing a plasma is referred to as plasma-enhanced CVD or PECVD. The plasma is utilized to provide energy to the process and enhance the deposition quality and/or deposition rate. Other plasma deposition processes also exist as are commonly understood by a person of ordinary skill in the art.
During plasma processing of a semiconductor substrate, it is often useful to apply an accelerating voltage to the surface of the substrate. The accelerating voltage or substrate bias is utilized to accelerate ions or other charged particles within the plasma to the substrate surface. In an etch process, the charged plasma particles are attracted to the substrate surface to actually bombard the surface and provide the etch as discussed above. In a deposition process, such as PECVD, the energy provided by such charged particle bombardment may be utilized to further enhance the deposition rate or to enhance the deposition quality of the material layer which is being deposited.
Generally, biasing of the substrate in plasma-enhanced etch and deposition processes is accomplished by capacitively coupling an RF field from RF biased electrodes in the processing chamber, through the substrate, and to the exposed substrate surface which is to be etched, or which is to receive a deposited material layer. Specifically, the electrodes, which are positioned within a susceptor or substrate support, are biased with an RF power supply to create an RF field. The RF field is then capacitively coupled through the susceptor and substrate to create a relatively uniform DC bias potential across the upper exposed substrate surface. The substrate surface DC bias, in turn, affects the plasma, as discussed above, to enhance the etch or deposition process.
Within a plasma processing system, the plasma will usually have particular non-uniformities associated therewith. For example, the plasma density is often greatest in the center of the plasma, due to edge effects proximate the sides of the processing chamber. The non-uniformities in the plasma may translate to discrepancies within the etch and deposition processes in which the plasma is utilized. For example, an undesirable variation in etch rate may occur wherein the etch rate proximate the center of the substrate is greater than the etch rate proximate the outer edges of the substrate. Furthermore, within a plasma-enhanced deposition process, the deposition may be affected proximate the center of the substrate differently than at the edge of the substrate thus creating a non-uniform deposition layer and a non-uniform deposition rate radially across the substrate.
Attempts have been made in the art to address such plasma non-uniformities in a plasma processing system. For example, U.S. Patent Application entitled xe2x80x9cImproved Apparatus and Method for Plasma Processing of a Substrate Utilizing an Electrostatic Chuck, xe2x80x9d U.S. Ser. No. 09/565,606, filed May 4, 2000, discloses a plasma processing system which selectively adjusts the bias on the substrate to offset plasma non-uniformities in the system; that application is incorporated herein by reference in its entirety. While that system improves the overall plasma process, it has been difficult to achieve precise selectivity in varying the substrate bias. Therefore, it is an objective of the present invention to provide more precise adjustments to the substrate bias in a plasma processing system for addressing non-uniformities and other vagaries in the plasma.
In accordance with another aspect of the invention, it is desirable to provide precise bias control even in a system utilizing an electrostatic chuck. Particularly during integrated circuit fabrication, the substrate being processed is supported within the processing chamber by a substrate support or susceptor. Oftentimes, the substrate is physically secured on the susceptor during processing, such as to improve heat transfer between the substrate and susceptor. One way of securing a substrate involves the use of an electrostatic chuck (ESC), which uses an applied DC bias to the substrate to electrostatically attract and secure the substrate to the susceptor. Electrostatic chucks are known in the art with suitable designs being shown in U.S. Patent Application entitled xe2x80x9cImproved Apparatus and Method for Plasma Processing of a Substrate Utilizing an Electrostatic Chuck,xe2x80x9d U.S. Ser. No. 09/565,606, filed May 4, 2000, noted above, and in U.S. Pat. No. 5,117,121, which patent is also incorporated herein by reference. Electrostatic chucks will usually use the same electrodes as are used to bias the substrate. This practice has made precise measurement of the substrate surface bias levels even more difficult due to the effect of the electrostatic clamping voltage on such measurement. Therefore, it is a further objective of the invention to provide more precise biasing of a substrate to address plasma non-uniformities within a processing system utilizing an electrostatic chuck.
Systems have been proposed for measuring substrate bias surface levels for an RF induced DC bias on a substrate. One such system is the subject of a U.S. Patent Application entitled, xe2x80x9cImproved Apparatus and Method for Monitoring Substrate Biasing During Plasma Processing of a Substrate, U.S. Ser. No. 09/580,824 and filed on May 26, 2000, which application is incorporated herein by reference. While that application discloses an apparatus and methodology for measuring the substrate bias, it is affected by the level of DC current that is available to the measuring circuit. Particularly, suitable DC current levels may not be available for proper measurements.
The dielectric material utilized between the RF electrodes of a susceptor and the substrate traditionally has a very high resistivity. Such high resistivity results in a low available DC current at the susceptor, and therefore, makes use of an amplification circuit necessary for RF bias measurements, as set forth in the above-referenced application. For example, the resistivity of dielectric components of the susceptor may represent resistances in the range of tens of MegaOhms. Such low DC current issues and measurement difficulties must be addressed on susceptors or substrate supports which utilize mechanical clamping structures to hold the wafer thereon, as well as susceptors which utilize electrostatic clamps. Therefore, it is an objective of the invention to provide for measurement of the RF-induced DC bias on a substrate, in order that the bias may be selectively adjusted for desirable processing results.
It is still another objective of this invention to address the above-discussed objectives without adversely affecting the desired biasing of the substrate surface which is necessary for plasma processing.
These objectives and other objectives will become more readily apparent from the further description of the invention below.
The present invention provides for accurate measurement of the RF-induced DC bias on the electrodes of a substrate support so that the DC bias may be selectively adjusted to obtain the desirable effect on the plasma within the processing system. To that end, a plurality of electrodes are coupled to a substrate support, such as by being embedded in the substrate support. The electrodes are each positioned proximate the supporting surface of the substrate support and are electrically isolated from one another. For example, the electrodes might be surrounded by a dielectric material. Alternatively, the entire substrate support might be made of a dielectric material and the electrodes might be embedded therein. An RF power source is coupled to each of the electrodes across a respective splitting capacitor for biasing the electrodes. The biased electrodes thereby develop an RF-induced DC bias thereon, and are operable for creating a DC bias on a substrate which is positioned on the supporting surface of the substrate support.
In accordance with one aspect of the present invention, a measurement circuit is utilized which includes a voltage measurement circuit electrically coupled to each of the electrodes at a point between that electrode and its respective splitting capacitor. The voltage measurement circuit is operable for measuring a voltage at the point, which voltage is associated with the electrode, and each electrode includes such a voltage measurement circuit. Another voltage measurement circuit is electrically coupled to the RF power source and is operable for measuring the voltage which is provided for the electrodes by the RF power source.
Based upon the difference between the power source measurement and the measurements across each splitting capacitor for an electrode, a voltage drop may be determined across each splitting capacitor. Reflective of that voltage drop, the DC bias associated with each of the electrodes is determined. The measurement circuit utilized for obtaining the voltage measurements may incorporate a processor for utilizing the measured values and determining the DC bias associated with the various electrodes.
More specifically, the present invention incorporates an inventive measurement circuit model of the processing system wherein the circuit model is configured in accordance with the principles of the invention to reflect the electrical characteristics of the various components or sub-systems of the overall processing system. Based upon the presumed and known electrical characteristics of those sub-systems, voltage drops across each of the sub-systems may be determined based upon the measured voltage values. For example, the inventive circuit model incorporates a circuit leg for each of the electrodes. The RF current through each leg is then determined based upon the voltage measurements. The present invention utilizes the electrical characteristics for each component or sub-system, in combination with the determined RF current, in order to establish a voltage drop associated with each component or sub-system. The presumed electrical characteristics and known electrical characteristics are utilized to determine the voltage drop across the plasma sheath associated with each of the electrodes, and therefore, to determine the bias voltage associated with each of the electrodes.
In accordance with one aspect of the present invention, the electrical characteristics of the substrate in the processing system, as well as the characteristics of the dielectric material associated with the substrate support, are estimated and calculated based upon the shape and qualities of the substrate support dielectric material and also the electrical characteristics associated with each material layer of the substrate being processed.
Further features of the invention and its various advantages are set forth in greater detail below.